Nano-Size Lead-Free Piezoceramic Powder and Method of Synthesizing the Same

ABSTRACT

A nano-size lead-free piezoceramic powder and a method of mechanochemically synthesizing the same are provided. The nano-size lead-free piezoceramic powder can have a basic component of (K x Na 1-x )NbO 3 , where x ranges from 0 to 1. A weight ratio of a milling ball to a raw powder can be set, and then the milling ball and the raw powder can be provided into a milling container at the set ratio. Nano-size lead-free piezoceramic powder can be mechanochemically synthesized using a high-energy ball mill device.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims the benefit under 35 U.S.C. § 119 toKorean Patent Application No. 10-2006-0123978, filed Dec. 7, 2006, whichis hereby incorporated by reference in its entirety.

BACKGROUND

In general, piezoceramics generate a voltage when pressure is appliedand undergo a mechanical shape change when an electric field is applied.Also, piezoceramics are materials in which conversion between mechanicaland electrical energies is highly efficient.

Piezoceramics are used in various industrial fields. Particularly, theuse of piezoceramics is increasing in fields such as electronic devices,medical equipment, and military supplies. Representative examples of theuse of piezoceramics include a medical ultrasonic sensor, a preciseposition controller, a piezo pump and valve, and various actuators.

However, because the currently-used piezoceramics are tertiary orquaternary ceramics, such as Pb(Zr,Ti)O₃-based compositions orPb(Mg_(1/3)Nb_(2/3))TiO₃-based compositions, which contain lead as amain element, these piezoceramics can cause serious problems. Forexample, in a process of fabricating the piezoceramics, a large amountof PbO is volatilized, which creates environmental pollution. Also,discarded components containing piezoceramics may cause ground pollutionand water pollution, which results in lead poisoning in the human body.

Therefore, it is necessary to substitute the existing lead-basedpiezoceramics with lead-free piezoceramics to avoid use of lead that isharmful to the human body and the environment.

There are several methods of fabricating lead-free piezoceramics.Examples of these methods are disclosed in Korean Patent Laid-OpenPublication No. 10-2004-0054965 and Japanese Patent Laid-OpenPublication No. 2006-06260. In each of the two publications, a mixtureof raw powder is ground/calcined to fabricate a first powder, and thenthe primary powder is ground/calcined to fabricate a phase-synthesizedsecond powder. Also, in Japanese Patent Laid-Open Publications Nos.2000-31664 and 2004-115293, a method of developing a composition oflead-free piezoceramics and adding a sintering aid such as CuO toincrease a sintering property is disclosed.

However, all of the above methods inevitably require a calcining processperformed at a high temperature ranging from 600° C. to 1000° C. tosynthesize the lead-free piezoceramic powder. Thus, the powdersynthesized by the high-temperature calcining process necessarily has asize greater than hundreds of nanometers.

Consequently, the methods cannot be used to synthesize piezoceramicpowder having a size on the order of tens of nanometers or less.

Also, to obtain a high-density sintered compact, the sinteringtemperature must be increased or a sintering aid, such as CuO, must beadded. However, the amount by which the sintering temperature may beincreased is limited because a high sintering temperature may causevolatilization of elements having high volatility such as Na and K.Thus, the characteristics of the piezoceramics may be deteriorated.

BRIEF SUMMARY

The present invention is directed to a nano-size lead-free piezoceramicpowder and a method of synthesizing the same that substantially obviatesone or more limitations and disadvantages of the related art.

Embodiments of the present invention provide a nano-size lead-freepiezoceramic powder that has a basic composition of (K_(x)Na_(1-x))NbO₃,where x ranges from 0 to 1, can be synthesized by a mechanochemicalmethod using a high-energy ball mill device, and thus can improve thesintering density even at a low sintering temperature in a subsequentsintering process.

Another embodiment of the present invention provides a method ofmechanochemically synthesizing nano-size lead-free piezoceramic powder.

Additional features of the present invention will be set forth, in part,in the description which follows and, in part, will become apparent tothose having ordinary skill in the art upon examination of the followingor may be learned from practice of the invention.

To achieve embodiments of the present invention, as exemplified andbroadly described herein, a method of mechanochemically synthesizingnano-size lead-free piezoceramic powder having a basic component of(K_(x)Na_(1-x))NbO₃, where x ranges from 0 to 1, is provided. In anembodiment, the method can include: setting a weight ratio of a millingball to a raw powder; providing the milling ball and the raw powder intoa milling container at the weight ratio; and mechanochemicallysynthesizing the nano-size lead-free piezoceramic powder using ahigh-energy ball mill device.

Also, materials of a milling ball and a milling container of ahigh-energy ball mill device, as well as the milling time, can becontrolled, so that lead-free piezoceramic powder of a size on the orderof tens of nanometers or less can be synthesized without a heattreatment such as a calcining process. Controlling the materials andmilling time can also be lead to the synthesis of lead-free piezoceramicpowder with various compositions.

The nano-size lead-free piezoceramic powder can have improvedcharacteristics because a sintering temperature in a subsequentsintering process can be lowered and, thus, volatilization of elementshaving strong volatility such as Na and K, which can be present in thelead-free piezoceramic powder, can be minimized.

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and are intended to provide further explanation of theinvention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing synthesis behavior as a function of millingtime in mechanochemically synthesizing NaNbO₃, a nano-size lead-freepiezoceramic powder, according to an embodiment of the presentinvention.

FIG. 2 is an electron microscope image showing a fine structure ofNaNbO₃, which can be mechanochemically synthesized according to anembodiment of the present invention.

FIG. 3 is a graph showing synthesis behavior as a function of millingtime when (K_(0.5)Na_(0.5))NbO₃, which is a nano-size lead-freepiezoceramic powder, is mechanochemically synthesized according to anembodiment of the present invention.

FIG. 4 is an electron microscope image showing a fine structure of(K_(0.5)Na_(0.5))NbO₃ which can be mechanochemically synthesizedaccording to an embodiment of the present invention.

DETAILED DESCRIPTION

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. The invention may, however, be embodied in many differentforms and should not be construed as being limited to the embodimentsset forth herein.

In an embodiment of the present invention, KNN-based lead-freepiezoceramic powder having a basic composition expressed as(K_(x)Na_(1-x))NbO₃, where x ranges from 0 to 1, can bemechanochemically synthesized. First, raw powder can be weighed to adesired composition ratio and provided into a milling container. Themechanochemical synthesis can then be performed at or near roomtemperature using a high-energy ball mill device with a milling ball.

The high-energy ball mill device can be, for example, a vibratory/shakermill, a planetary mill, or an attrition mill. In an embodiment, a shakermill having a speed of about 900 rpm to about 1200 rpm can be used.

A shaker mill is a vibratory mill, which 3-dimensionally (3-D) vibratesin vertical and horizontal directions. During synthesis, the shaker millcan scatter and grind raw powder into nanoscale fine particles by 3-Dvibration.

The high-energy ball mill device can include a milling container and amilling ball. The milling container and the milling ball can each beformed of, for example, a zirconia-based material, an iron-basedmaterial, or a tungsten carbide-based material. The milling containerand the milling ball can be selected to be suitable for the type ofinput raw powder.

In an embodiment, a weight ratio of the milling ball to raw powderprovided into the milling container can be from about 10:1 to about50:1. The weight ratio can be set to a suitable value depending onmaterial of the milling container, materials of the milling ball, andthe type of raw powder.

If the weight ratio of the milling ball to the raw powder is less thanabout 10:1, the energy of collision between milling balls and the energyof collision between the milling ball and the milling container canbecome low during a ball-milling operation using the high-energy ballmill device. Thus, disadvantageously, the scattering and grinding effectof the raw powder can be deteriorated significantly. If the weight ratioof the milling balls to the raw powder is greater than about 50:1, theamount of raw powder provided into the milling container can be small,which can lead to an undesirable lowering of the probability that theraw powder is placed between the milling balls or between the millingball and the milling container.

The milling time of the high-energy ball milling can vary based on thetype of raw powder being used, the weight ratio between the milling balland the raw powder, and the materials of the milling ball and themilling container. In an embodiment, high-energy ball milling by thehigh-energy ball milling device can be performed for at least about 10minutes.

Raw powder and balls can be provided into the milling container of thehigh-energy ball milling device, and the ball milling can be performedwithout adding any separate liquid additive. Thus, in an embodiment, dryhigh-energy ball milling can be performed.

When the raw powder is provided into the milling container together withthe milling ball, lithium (Li), magnesium (Mg), calcium (Ca), strontium(Sr), barium (Ba), lanthanum (La), silver (Ag), copper (Cu), arsenic(As), selenium (Se), bismuth (Bi), tantalum (Ta), antimony (Sb),titanium (Ti), tungsten (W), or any combination thereof can be added. Inan embodiment, lead-free piezoceramic powder having a compositionobtained by adding Li, Mg, Ca, Sr, Ba, La, Ag, Cu, As, Se, Bi, Ta, Sb,Ti, or W to the basic composition of KNN-based lead-free piezoceramicpowder can be synthesized. Here, the basic composition of the KNN-basedlead-free piezoceramic powder is (K_(x)Na_(1-x))NbO₃, where x rangesfrom 0 to 1.

Examples of the present invention will now be described in detail. Thefollowing examples of the present invention are used only to describethe present invention, and it will be obvious to those skilled in theart that the scope of the present invention is not limited thereto.

EXAMPLE 1

Na₂CO₃ and Nb₂O₅ are prepared as ceramic raw powder and weighed suchthat a composition of a compound synthesized after a reaction becomesNaNbO₃. Then, the resulting ceramic raw powder is placed in azirconia-based milling container, together with tungsten carbide-basedmilling balls.

The weight ratio of the tungsten carbide-based milling ball to theceramic raw powder is set to about 30:1. High-energy ball milling isperformed by a shaker mill for about 20 hours, thereby fabricatingnano-size NaNbO₃ by a mechanochemical reaction.

In FIG. 1, phase synthesis behavior over milling time is illustrated.Referring to FIG. 1, initial raw powder includes Na₂CO₃ and Nb₂O₅.Respective peaks representing Na₂CO₃ and Nb₂O₅ gradually decrease overmilling time, while a peak representing NaNbO₃ being mechanochemicallysynthesized gradually increases. Thus, three phases exist at the sametime after about one hour of the high-energy ball milling.

Most of the phases are synthesized into NaNbO₃ after about two hours ofhigh-energy milling. Although the milling is performed for about 20hours, no other phases are generated, and only NaNbO₃ phase exists.

FIG. 2 illustrates an electron microscope image of lead-free ceramicpowder synthesized mechanochemically through about two hours ofhigh-energy ball milling. Referring to FIG. 2, the synthesized lead-freeceramic powder is formed as lumped particles that are each about 10nanometers to about 20 nanometers in size.

EXAMPLE 2

Na₂CO₃, K₂CO₃, and Nb₂O₅ are prepared as ceramic raw powder and weighedsuch that the composition of the compound synthesized after a reactionis (K_(0.5)Na_(0.5))NbO₃. Then, the resulting ceramic raw powder isplaced in a zirconia-based milling container. Here, tungstencarbide-based milling balls are provided into the milling container,together with the ceramic raw powder.

The weight ratio of the tungsten carbide-based milling balls to theceramic raw powder is set to about 30:1. The high-energy ball milling isthen performed using a shaker mill for about 20 hours, therebyfabricating nano-size (K_(0.5)Na_(0.5))NbO₃ by a mechanochemicalreaction.

In FIG. 3, the phase synthesis behavior over milling time isillustrated. Referring to FIG. 3, initial raw powder includes K₂CO₃,Na₂CO₃, and Nb₂O₅, and respective peaks representing K₂CO₃, Na₂CO₃, andNb₂O₅ gradually decrease over the milling time while a peak representing(K_(0.5)Na_(0.5))NbO₃ being mechanochemically synthesized graduallyincreases.

Most of the phases are synthesized into (K_(0.5)Na_(0.5))NbO₃ afterabout 9 to 10 hours of high-energy ball milling. Although thehigh-energy balling is performed for about 20 hours, no other phases aregenerated, and only the phase of (K_(0.5)Na_(0.5))NbO₃ exists.

FIG. 4 illustrates an electron microscope image of powder that issynthesized mechanochemically through about 9 to 10 hours of high-energyball milling. Referring to FIG. 4, the synthesized powder is formed aslumped particles that are each about 10 nanometers to about 20nanometers in size.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present invention. Thus,it is intended that the present invention covers the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

1. A method of mechanochemically synthesizing a nano-size lead-freepiezoceramic powder, comprising: setting a weight ratio of a millingball to a raw powder; providing the milling ball and the raw powder intoa milling container at the weight ratio; and mechanochemicallysynthesizing the nano-size lead-free piezoceramic powder using ahigh-energy ball mill device; wherein the nano-size lead-freepiezoceramic powder comprises (K_(x)Na_(1-x))NbO₃, where x ranges from 0to
 1. 2. The method according to claim 1, wherein the nano-sizelead-free piezoceramic powder is mechanochemically synthesized at aboutroom temperature.
 3. The method according to claim 1, whereinmechanochemically synthesizing of the nano-size lead-free piezoceramicpowder comprises performing dry high-energy ball milling.
 4. The methodaccording to claim 1, wherein the milling ball comprises azirconia-based material, an iron-based material, or a tungstencarbide-based material.
 5. The method according to claim 1, wherein themilling container comprises a zirconia-based material, an iron-basedmaterial, or a tungsten carbide-based material.
 6. The method accordingto claim 1, wherein the high-energy ball mill device is avibratory/shaker mill, a planetary mill, or an attrition mill.
 7. Themethod according to claim 1, wherein the weight ratio of the millingball to the raw powder is from about 10:1 to about 50:1.
 8. The methodaccording to claim 1, wherein the weight ratio of the milling ball tothe raw powder is about 30:1.
 9. The method according to claim 1,wherein before mechanochemically synthesizing the nano-size lead-freepiezoceramic powder, lithium (Li), magnesium (Mg), calcium (Ca),strontium (Sr), barium (Ba), lanthanum (La), silver (Ag), copper (Cu),arsenic (As), selenium (Se), bismuth (Bi), tantalum (Ta), antimony (Sb),titanium (Ti), or tungsten (W) is added to the raw powder.
 10. Themethod according to claim 1, wherein mechanochemically synthesizing thenano-size lead-free piezoceramic powder using a high-energy ball milldevice is performed for about 20 hours.
 11. A nano-size lead-freepiezoceramic powder, comprising (K_(x)Na_(1-x))NbO₃, where x ranges from0 to 1; wherein the nano-size lead-free piezoceramic powder issynthesized by a method comprising: setting a weight ratio of a millingball to a raw powder; providing the milling ball and the raw powder intoa milling container at the weight ratio; and mechanochemicallysynthesizing the nano-size lead-free piezoceramic powder using ahigh-energy ball mill device.
 12. The nano-size lead-free piezoceramicpowder according to claim 11, wherein the nano-size lead-freepiezoceramic powder is mechanochemically synthesized at about roomtemperature.
 13. The nano-size lead-free piezoceramic powder accordingto claim 11, wherein mechanochemically synthesizing of the nano-sizelead-free piezoceramic powder comprises performing dry high-energy ballmilling.
 14. The nano-size lead-free piezoceramic powder according toclaim 11, wherein the milling ball comprises a zirconia-based material,an iron-based material, or a tungsten carbide-based material.
 15. Thenano-size lead-free piezoceramic powder according to claim 11, whereinthe milling container comprises a zirconia-based material, an iron-basedmaterial, or a tungsten carbide-based material.
 16. The nano-sizelead-free piezoceramic powder according to claim 11, wherein thehigh-energy ball mill device is a vibratory/shaker mill, a planetarymill, or an attrition mill.
 17. The nano-size lead-free piezoceramicpowder according to claim 11, wherein the weight ratio of the millingball to the raw powder is from about 10:1 to about 50:1.
 18. Thenano-size lead-free piezoceramic powder according to claim 11, whereinthe weight ratio of the milling ball to the raw powder is about 30:1.19. The nano-size lead-free piezoceramic powder according to claim 11,wherein before mechanochemically synthesizing the nano-size lead-freepiezoceramic powder, Li, Mg, Ca, Sr, Ba, La, Ag, Cu, As, Se, Bi, Ta, Sb,Ti, or W is added to the raw powder.
 20. The nano-size lead-freepiezoceramic powder according to claim 11, wherein mechanochemicallysynthesizing the nano-size lead-free piezoceramic powder using ahigh-energy ball mill device is performed for about 20 hours.